c/CEPB, a chicken transcription factor of the leucine-zipper C/EBP family

The gene encoding the chicken homologue of the rat CCAAT/Enhancer Binding protein C/EBPa (1, 2) was cloned and sequenced. The strategy consisted of the screening of a chicken liver cDNA library with a rat C/EBP clone yielding a partial cDNA (3), which in turn served as a probe to screen a chicken genomic DNA library. Of two independent clones, a 1686-bp region covering the C/EBP gene was sequenced. Both sequences are identical and contain a 971-bp open reading frame. A putative TATA box is present at 230 bp 5' and a putative polyadenylation signal at 285 bp 3' to the open reading frame. Comparative sequence analysis shows that the derived 324-amino acid chicken C/EBP sequence is highly similar (68.5%) to the 358-residue C/EBPa sequence of rat (Figure 1). The C-terminal moieties constituting the basic, DNA-binding and leucine zipper, dimerisation domains are virtually identical (94%). The N-terminal moieties are partially conserved (59%); three highly conserved regions, designated I, II and Ill (Figure 1), can be distinguished and may correspond to individual functional domains. Our conserved region I coincides with the N-terminal trans-acting sequence defined in rat C/EBPa. Regions II and III map in a region of which the function is not clear (4, 5). The conserved regions revealed in our investigations may help to further define the functional domains. Chicken C/EBP differs notably from the rat C/EBPa in not having the proline and glycine stretches lying between the conserved regions. In the 5' untranslated sequence of chicken C/EBP, we find a small open reading frame for a hypothetical peptide of 5 amino acids (MPGRL) separated by 7 nt from the C/EBP reading frame. A sequence potentially encoding a similar peptide (MPGEL) is present at exactly the same position in the rat C/EBPa gene. The cognate DNA sequence only shows moderate similarity (11/18), suggesting that conservation is imposed at the amino acid level. As shown earlier by cotransfection experiments with a rat C/EBPat expression vector, C/EBP is a potential factor involved in the activation of the liver-specific, estrogen inducible apoVLDL II gene (6). In view of the sequence differences found, it will be interesting to compare the activities of the homologous cC/EBP in transfection experiments. REFERENCES